Fuel cells are power generation systems that convert chemical energy generated from oxidation of fuel into electric energy. Fuel cells have a higher efficiency compared to other internal combustion engines, and are substantially free from emission of pollutants. Therefore, fuel cells have become the focus of attention as alternative energy technology.
In general, a fuel cell comprises an anode, a cathode and an electrolyte. Fuel cells may be classified into low-temperature fuel cells and high-temperature fuel cells depending on the electrolyte used therein. In the case of a low temperature fuel cell that operates at a temperature of 300° C. or lower, the fuel cell is necessary to show high catalyst reactivity and ion permeability at low temperature in order to obtain a desired level of energy. Therefore, an electrode catalyst as well as an electrolyte is a critical factor determining the overall quality of a fuel cell. Currently, most electrode catalysts for low temperature fuel cells are comprised of Pt/support (i.e. platinum loaded on a support), or non-supported platinum black. Herein, it is important to reduce the amount of platinum or to maximize the activity of platinum per unit weight, because of the high cost of platinum. To achieve this, it is necessary to increase the active region of a catalyst by controlling the size of the platinum particles carried on a support to a nano-scaled size. Meanwhile, when preparing a membrane electrode assembly, degradation of the quality caused by degradation of mass diffusion decreases as the thickness of a catalyst layer decreases. Therefore, it is necessary to reduce the amount of a support, so that a highly supported catalyst can be obtained. Further, it is necessary to provide a highly dispersed Pt/support catalyst comprising Pt microparticles.
Conventional methods for preparing Pt/support powder by loading Pt particles on a support are broadly classified into a precipitation method and a colloidal method.
The precipitation method is carried out in a liquid phase for the most part and is not significantly affected by process parameters. Therefore, the precipitation method is relatively simple, and is readily amenable to scale-up. However, it has the disadvantages of non-uniform dispersion of platinum particles and a relatively large size of particles. On the contrary, according to the colloid method, it is possible to obtain fine platinum particles with a narrow size distribution of 1.5-3 nm and to accomplish uniform dispersion of particles, when platinum is supported in an amount of 20˜40 wt % based on the total weight. However, the colloid method has the following problems: platinum particle size increases rapidly as the amount of Pt increases to 50 wt % or more; an additional hydrogen reduction step is required; and water should be introduced at quantities of 10 times or greater than the amount needed for the precipitation method. Therefore, an optimized method for preparing an electrode catalyst is needed, so as to simplify the processing steps and to obtain fine Pt particles.
Meanwhile, most supports developed for use in electrode catalysts have a specific surface area of 300 m2/g or less. Even in the case of a high-surface area support used in commercially available catalysts, the support generally has a surface are of 800 m2/g or less. When preparing an electrode catalyst by using the above support, the particle size of Pt decreases merely to a limited level, and Pt particles cannot be dispersed uniformly on the support surface. As a result, there is a serious problem in that the active region of the Pt catalyst cannot be increased sufficiently.